How to set color on triangle of a mesh
What seems to be different colors on some faces than on others is a function of the direction of the normals of the faces, and the location and energy level of the lamp(s). Faces where the normals of the faces are oriented so that you are looking at them head on in line with, or close to in line with the location of a lamp in the scene will have a more intense hue than faces that are oriented so that you seem to be looking at them on an oblique angle, or at a point that is distant from the line between the location of the lamp and the part of the image you are viewing.  

To get a more uniform color on the material, you may need to either change the lamps, add additional lamps to the scene, or both. Depending upon the scene, you might want to change the default lamp to type "sun", change it's elevation ("z" axis value) to something larger, perhaps 500 or 1000 Blender units, and change the energy to a higher value, too. Start, perhaps, with and energy of 250.